Make the test of jemalloc optional

Jemalloc is not supported on Windows (MSVC).  There's no way to run
tests of other crates without manually removing jemalloc dependencies
and integration tests.  This may lead some mistake (e.g. including
jemalloc removed `Cargo.toml` to the commit mistakenly) so I want to
make easier to ignore the test on Windows.

Now, the test will be run only when `jemalloc` feature is enabled.

Note that the feature is default feature, since completely ignoring the
test by default will forget running the test forever.  So `cargo test`
will run the test usually.  To ignore the test, run `cargo test
--no-default-features` to disable `jemalloc` feature.

Author: statiolake

Cargo.toml

@@ -9,6 +9,10 @@ edition = "2018"
 name = "main"
 path = "src/main.rs"
 
+[features]
+jemalloc = ["jemalloc-ctl", "jemallocator"]
+default = ["jemalloc"]
+
 [dependencies]
 # AtCoder 2019年言語アップデート以降に使用できるクレート
 
@@ -70,8 +74,14 @@ ndarray = "=0.12.1"
 nalgebra = "=0.18.1"
 
 # 代替ヒープアロケータ。条件によってはシステムアロケータより速いことも
-jemallocator = "=0.3.2"
-jemalloc-ctl = "=0.3.3"
+[target.'cfg(not(windows))'.dependencies]
+jemallocator = { version = "=0.3.2", optional = true }
+jemalloc-ctl = { version = "=0.3.3", optional = true }
+
+[[test]]
+name = "jemallocator"
+path = "tests/test_jemallocator.rs"
+required-features = ["jemalloc"]
 
 # ---------------------------------------------------------------------